The present invention concerns the cold forging or cold hammering of objects of steel or other metals, and the invention concerns more specially a mandrel for the cold forging of profiled tubes or cylinders etc. Profiled means in this case any profiled form such as axial grooves or splines, various kinds of threads, threads combined with axial grooves etc.
Internal profiles in tubes or cylinders have earlier been produced by cutting methods such as turning, shaping, slotting etc. The production of internal profiles in tubes and cylinders etc. has of late even been accomplished by cold forging, whereby a tube or cylinder is forged around a mandrel, which after the process is removed from the tube or cylinder.
Cold forging involves several advantages compared to cutting methods. By cold forging a finer surface finish can be achieved than with cutting methods, the material is harder as a result of the fact that no material fibers are cut off, internal stresses from earlier handling are eliminated, the profiles can be formed to extremely fine tolerances, the cold forged products are produced with an even and high quality etc.
Cold forging around a mandrel however does present some problems. As cold forging is normally effected by a successive working of a tube or cylinder from one end to the other by using forging hammers, the workpiece will during forging be worked with great force into the mandrel's grooves, pits or such-like, so that the mandrel and workpiece are forced into very close contact with each other. By hammer working the workpiece from one end to the other, the workpiece will lengthen in a direction which is the same as the forging hammers' direction of movement along the workpiece. This leads to the rise of great axial forces between the workpiece and the mandrel. This in turn leads to problems which can arise when removing the mandrel from the final product.
When forming axial grooves, the mandrel must be removed axially from the forged product, and when forming internal threads or other spiral formations, the mandrel must be removed from the product by a screwing motion. Special problems arise in case where the workpiece, apart from axial grooves or threads, is also formed with a circular or purely radial rib. This occurs for example with joining sleeves etc. where a rib ensures that rods, for example drilling rods, cannot be pushed or screwed in further than to a given position in the tube or sleeve. In this case the mandrel obviously cannot be screwed or drawn out of the sleeve and therefore the production of such sleeves has not earlier been possible by cold forging in only one working step.
The present invention intends to eliminate the said problem and to provide a mandrel for the production of internally profiled tubes or cylinders, which allows a relatively simple removal of the mandrel from the final forged product and which even makes it possible in only one working step, to produce an internal profile comprising a circular or purely radial rib in the cylinder.